Magnetic random access memory with selective toggle memory cells

ABSTRACT

A toggle MTJ is disclosed that has a SAF free layer with two or more magnetic sub-layers having equal magnetic moments but different anisotropies which is achieved by selecting Ni ˜0.8 Fe ˜0.2  for one sub-layer and CoFeB or the like with a uni-axial anisotropy of 10 to 30 Oe for the higher anisotropy sub-layer. When a field is applied at &lt;10° angle from the easy axis, magnetic vectors for the two sub-layers rotate to form different angles from the easy axis. A method is also described for selectively writing to bits along a word line that is orthogonal to bit line segments and avoids the need to “read first”. A bipolar word line pulse with two opposite pulses separated by a no pulse interval is applied in the absence of a bit line pulse to write a “0”. A bit line pulse opposite the second word line pulse writes a “1”.

This is a Divisional application of U.S. patent application Ser. No.11/340,989, filed on Jan. 27, 2006, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

FIELD OF THE INVENTION

The invention relates to an MRAM structure having a magnetic regioncomprised of a synthetic anti-ferromagnetic (SAF) free layer in whichsub-layers thereof are anti-parallel coupled and have equal magneticmoments but unequal anisotropies that enables writing to selected MTJcells without a “read first” requirement.

BACKGROUND OF THE INVENTION

Magnetic random access memory (MRAM) that incorporates a magnetictunneling junction (MTJ) as a memory storage cell is a strong candidateto provide a high density, fast (1-30 ns read/write speed), andnon-volatile storage solution for future memory applications. An MRAMarray is generally comprised of an array of parallel first conductivelines on a horizontal plane, an array of parallel second conductivelines on a second horizontal plane spaced above and formed in adirection perpendicular to the first conductive lines, and an MTJ formedat each location where a second conductive line crosses over a firstconductive line. A first conductive line may be a word line while asecond conductive line is a bit line or vice versa.

Referring to FIG. 1, a conventional MRAM structure 1 is shown in whichan MTJ 11 is formed between a first conductive line 10 and a secondconductive line 12. In this example, the first conductive line is a wordline and the second conductive line is a bit line although the terms areinterchangeable. A conductive line may also be referred to as a digitline, row line, data line or column line. The word line 10 and bit line12 are used for writing data into the MTJ 11. The MTJ consists of astack of layers with a configuration in which two ferromagnetic layersare separated by a thin non-magnetic insulating layer such as Al₂O₃,AlN_(x)O_(y), or MgO which is called a tunnel barrier layer. In aso-called bottom spin valve configuration, the bottom portion 13 is acomposite layer with a lower seed layer, a middle anti-ferromagnetic(AFM) layer, and an upper pinned layer (first ferromagnetic layer). TheAFM layer is exchange coupled to the pinned layer and thereby fixes themagnetization (magnetic moment) direction of the pinned layer in apreset direction. Above the pinned layer is the tunnel barrier layer 14.The second ferromagnetic layer is a free layer 15 on the tunnel barrierlayer and has a magnetization direction that can be changed by externalmagnetic fields. To maintain data against erasure or thermal agitation,an in-plane uni-axial magnetic anisotropy is needed for the free layer15. The top layer in the MTJ 11 is generally a cap layer 16.

During a write operation, an electrical current I₁ in bit line 12 and acurrent I₂ in word line 10 yield two magnetic fields on the free layer15. The magnetic fields conform to a right hand rule so that a firstfield is generated along a first axis (easy axis) in the plane of thefree layer and a second field is produced in a direction orthogonal tothe first axis and along a hard axis in the free layer. In response tothe magnetic fields generated by currents I₁ and I₂, the magnetic vectorin the free layer is oriented in a particular stable direction thatrepresents a memory state. The resulting magnetic vector orientationdepends on the direction and magnitude of I₁ and I₂ and the propertiesand shape of the free layer 15. Generally, writing a zero “0” requiresthe direction of either I₁ or I₂ to be different than when writing a one“1”. Thus, the magnetization direction of the free layer may be switchedfrom a “+x” to a “−x” direction, for example, that corresponds to achange in the memory state from a “0” to a “1” or vice versa.

The magnitude of the magnetic field used to switch the magnetic vectoris proportional to the amplitude of I₁ and I₂. The amplitude of I₁ andI₂ is on the order of several milli-Amperes for most designs. As thesize of MTJs shrinks to 0.1 micron or smaller, the switching fields areexpected to become larger and switch transistors will demand a largeramount of chip area. It is desirable to reduce power consumption andthis adjustment is achieved in some cases by increasing the field percurrent ratio of the conductor. A prior art method for increasing thefield per current ratio is to provide a magnetic liner or cladding layeron one or more sides of a conductive line. Examples of cladding layersare described by Naji et al. in “A low power 1 Mbit MRAM based on ITIMTJbit cell integrated with Copper Interconnects”, VLSI Conf. (2002).

The typical writing scheme is a “half select” scheme where a bit lineand word line each generate half the required write field for switchingthe selected MTJ cell. However, the energized word and bit lines reducethe magnetic reversal energy barrier in the other cells along theirrespective word and bit lines. This condition makes these“half-selected” cells more susceptible to having their magnetic statesunintentionally switched when the selected cell is written.

In U.S. Pat. No. 6,335,890, an architecture for selectively writing oneor more magnetic memory cells in a MRAM device comprises at least onewrite line including a global write line conductor and a plurality ofsegmented write line conductors connected thereto. The global write lineconductor is substantially isolated from the memory cells.

An MRAM with a MTJ cell structure and switching mechanism that does notsuffer from the half select problem of the conventional MRAM has beenproposed by Motorola. This “Savtchenko” cell structure and switchingmechanism is described in U.S. Pat. No. 6,545,906 and by M. Durlam etal. in “A 0.18 micron 4 Mb Toggling MRAM”, IEDM Technical Digest 2003,Session 34, paper #6. In this type of MRAM, the MTJ cell's ferromagneticfree layer is a synthetic anti-ferromagnet (SAF) that may be amultilayer of two ferromagnetic sublayers of nearly identical magneticmoment, separated by an anti-ferromagnetic coupling layer that maintainsan anti-parallel alignment of the moments of the two sublayers. In a SAFfree layer, the sublayer which directly contacts the MTJ tunnel barrierlayer is the sensing layer. The pinned layer on the opposite side of thebarrier layer is the reference layer. When the sensing layer and pinnedlayer magnetization directions are parallel, the MTJ cell has lowresistance, and when the magnetization directions are anti-parallel, thecell has a high resistance.

The Savtchenko type of MRAM uses two orthogonal writing or programminglines, but with the MTJ cell's axis aligned 45 degrees to each of thelines. The SAF free layer responds to the applied magnetic fieldsdifferently than a conventional single ferromagnetic free layer. Writingoccurs by a process called “toggle” writing in which a two phaseprogramming pulse sequence incrementally rotates the SAF free layermoment or magnetization direction 180 degrees so the MRAM is sometimescalled a “toggling” MRAM and the memory cell a “toggle” cell. Because ofthe cell's 45 degree angle to the programming lines and its fieldresponse, the field from a single programming line cannot switch themagnetization of a half selected cell and thereby results in an MRAMwith enhanced cell selectivity.

The conventional toggling writing process always changes the selectedcell, independent of the sensing layer magnetization direction since thetwo magnetic sub-layers in the SAF are symmetrical. The toggling MRAM isa “read before write” MRAM which means all the toggle memory cells haveto be read first to find their magnetic states and determine whethertoggling writing is needed. This “read before write” schemesignificantly reduces the write cycle time.

However, it would be desirable to further reduce the write cycle time byimplementing a new toggle MTJ cell design to enable a direct writeprocess of selected cells to desired states. A direct writing schemewithout the need to know the previous magnetic states of the selectedcells would greatly improve the writing speed.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a selectivetoggling MRAM structure in which MTJ cells can be selectively written towithout first employing a read process to determine their memory states.

A further objective of the present invention is to provide a MTJ havinga SAF free layer in which sub-layers thereof are anti-parallel coupledand have essentially equal magnetic moments but unequal anisotropies.

Yet another objective of the present invention is to provide a writeprocess for the toggle MTJ cells having a SAF free layer in whichsub-layers thereof are anti-parallel coupled and have essentially equalmagnetic moments but different anisotropies.

These objectives are achieved in an MRAM structure comprised of an MTJthat is sandwiched between a first conductive line and second conductiveline hereafter referred to as bit line and word line, respectively. Inone aspect, there is a bit line having a top surface that contacts thebottom of the MTJ and a word line aligned orthogonally to the bit linethat has a bottom surface in contact with the top surface of the MTJ.The MTJ may have a bottom spin valve configuration wherein a seed layer,AFM layer, pinned layer, tunnel barrier, free layer, and cap layer aresequentially formed on the bit line. Preferably, the free layer has asynthetic anti-ferromagnetic (SAF) configuration wherein a firstsub-layer, coupling layer, and second sub-layer are sequentially formedon the tunnel barrier. A key feature is that the first sub-layer andsecond sub-layer have essentially the same magnetic moment but differentanisotropies. The different anisotropies result from selecting differentmaterials for the two sub-layers. The first sub-layer preferably has ahigher anisotropy and is made of soft magnetic material such as CoFeB,CoNiFe, CoFeB/NiFe, CoNiFe/NiFe, or is comprised of two soft magneticsub-sublayers that are coupled by Ru as in NiFe/Ru/NiFe. The secondsub-layer has a lower anisotropy and is comprised of a soft magneticmaterial such as Ni_(˜0.8)Fe_(˜0.2).

Alternatively, the SAF free layer may be comprised of four sub-layerseach having the same magnetic moment. Preferably, the outer sub-layerthat is the greatest distance from the tunnel barrier has a largeranisotropy than the inner three sub-layers. Moreover, each of theadjacent sub-layers are anti-ferromagnetically coupled through acoupling layer such as Ru and the two inside sub-layers closest to thetunnel barrier layer are strongly anti-parallel coupled.

In one embodiment, the easy axis of the SAF free layer is parallel tothe bit line and normal to the word line. For this design, writing toselected MTJ cells in a selective toggle mode comprises applying acurrent with a positive pulse followed by a negative pulse (or anegative pulse followed by a positive pulse) along the selected wordline that contacts the selected MTJ cells. To write “0” in each MTJ cellalong the selected word line, no current flows through bit lines thatcontact the selected MTJ cells (bits) and the second pulse of the wordline current writes the bits. In order to write “1” in each bit alongthe selected word line, a bit line current pulse overlays with thesecond pulse of the word line current. When the second word line currentpulse is negative, the bit line pulse is positive. In this scheme, thecombined magnetic fields generated by the second word line current pulseand bit line current pulse is sufficiently larger than the spin-flopfield for the SAF free layer and thereby avoids writing a “0”.

In a second embodiment, the easy axis of the SAF free layer bisects theangle formed by the intersection of a bit line and word line. The anglebetween the easy axis and bit line is the same as the angle between theeasy axis and word line and may range from about 45° to 60°. During theselective write process, the sum of the fields generated by the wordline current and bit line current is larger than its spin-flop fieldwith sufficient margin and is parallel to the easy axis (+x) directionfor writing “1” and in the opposite direction (−x) direction for writing“0”. The falling edges of the word line current pulse and bit linecurrent pulse are matched to overlap each other and use either longercurrent falling time or a multi-step falling edge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an oblique view showing a conventional MRAM structure in whichan MTJ is formed between a first conductive line and a second conductiveline.

FIG. 2 is a time sequence diagram that illustrates how the magneticvectors for two sub-layers of a SAF free layer in a conventional toggleMRAM are switched by sequentially applying magnetic fields in the “+x”direction followed by the “x+y” direction, “y” direction, and finallysetting the applied field back to zero.

FIG. 3 a is a cross-section of a toggle MTJ cell in which the free SAFlayer has two sub-layers according to one aspect of the presentinvention.

FIG. 3 b is a cross-section of a free SAF layer portion of a toggle MTJcell having four free sub-layers according to another aspect of thepresent invention.

FIGS. 4 and 5 are diagrams that show the rotation of magnetic vectorsfor the sub-layers of the SAF free layer relative to the easy axis ofthe MTJ cell according to the present invention when a field with alarge angle is applied away from the easy axis.

FIGS. 6 and 7 a are diagrams showing the rotation of magnetic vectorsfor sub-layers of the SAF free layer relative to the easy axis in atoggle MTJ cell according to the present invention when a field isapplied with a small angle away from the easy axis.

FIG. 7 b is a diagram similar to FIG. 7 a except an embodiment isdepicted wherein there are four sub-layers in the SAF free layer andmagnetic vectors for each sub-layer rotate clockwise in response to anapplied field.

FIG. 8 shows a simulation result for flip-flop field as a function offield angle away from the easy axis in a toggle MTJ cell according tothe present invention.

FIG. 9 is a top-view of a selective toggle MTJ cell having the easy axis(or anisotropy) of the two sub-layers of the SAF free layer alignedalong a write bit line according to one embodiment of the presentinvention.

FIG. 10 is an enlarged top view of FIG. 9 that shows all bits (MTJcells) along a selected word line are written to during the same writecycle.

FIG. 11 is a diagram showing the bipolar word line current pulse used towrite a “1” memory state to the selected MTJ cells in FIG. 10.

FIG. 12 is a diagram showing the bipolar word line current pulse and bitline current pulse used to write a “0” memory state to the selected MTJcells in FIG. 10.

FIG. 13 is a top-view of a selective toggle MTJ cell wherein the writeword line and write bit line have the same 45 to 60 degree angle awayfrom the cell easy axis according to a second embodiment of the presentinvention.

FIG. 14 is a diagram showing the direction of magnetic fields (relativeto the easy axis) used to write “1” or “0” memory states to the MTJ cellin FIG. 13.

FIGS. 15 and 16 are diagrams showing matched word line current and bitline current pulses having “longer current falling time” and “multi-stepfalling edges”, respectively, which are used to write the toggle MTJcell depicted in FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an MRAM structure having a toggle MTJ celldesign wherein a SAF free layer has at least two sub-layers that areanti-parallel coupled and have essentially equal magnetic moments butdifferent anisotropies. The drawings are provided by way of example andare not intended to limit the scope of the invention. Although certaindrawings depict a bit line formed below a MTJ and a word line above theMTJ, the designation for bit line and word line may be reversed.Moreover, the terms bit line and word line may be interchanged withother terms such as column line, row line, data line, and digit line.The MTJ may be a top MTJ, a bottom MTJ, or a multilayer MTJ asappreciated by those skilled in the art. “Write word line” and “writebit line” are terms that identify the word line and bit line used towrite a certain MTJ cell. Although only one MTJ cell is shown in somedrawings, it should be understood that there are a plurality of MTJcells in an array that has multiple rows and columns on an MRAM chip.The present invention is also a method of selectively writing a “0” or“1” memory state in one or more toggle MTJ cells described herein.

Referring to FIG. 2, the writing sequence for a conventional toggle MTJcell is illustrated. A top-down view of an elliptically shaped MTJ cell20 is depicted that is essentially centered at the intersection of anx-axis and y-axis that overlay on a first write line 21 and second writeline 22, respectively. The first write line 21 contacts the bottom ofthe MTJ cell 20 while the second write line 22 contacts the top of theMTJ cell. The MTJ cell 20 has a stack of layers (not shown) comprised ofa SAF free layer having a lower sub-layer or sub-layer 1 formed on atunnel barrier layer (not shown), a middle coupling layer, and an uppersub-layer or sub-layer 2 formed on the coupling layer. Sub-layer 1 has amagnetic vector 24 oriented along an easy axis 23 that bisects the angleformed by the x and y axes and sub-layer 2 has a magnetic vector 25oriented opposite to that of magnetic vector 24. The magnetic vectors24, 25 have essentially the same magnitude and the two sub-layers haveequal anisotropies. The easy axis of magnetization of the SAF free layeris the axis of anisotropy that can be induced by the shape of the MTJcell or by the deposition process such as deposition in an appliedmagnetic field or at an angle of incidence. For an ellipse, the easyaxis usually coincides with the long axis.

The orientations of the two magnetic vectors 24, 25 are shown duringtime intervals t₀-t₄. In a quiescent state during time interval t₀, nowrite line currents are applied and the magnetic vectors 24, 25 areoriented opposite each other along the easy axis 23. This condition mayrepresent a memory state “0”, for example. During interval t₁, a currentin write line 22 (WL-1) generates a magnetic field (Hy) in the +ydirection that causes both magnetic vectors 24, 25 to rotate in aclockwise manner. Note that the +y axis now bisects the angle formed bythe magnetic vectors 24, 25. During interval t₂, currents in write line21 and write line 22 (WL-2) generate magnetic fields Hy and Hx,respectively, that produce a net magnetic field along the easy axis 23(anisotropy direction) between the +x and +y axes. Again, the magneticvectors 24, 25 rotate in a clockwise manner (compared to their previousstate). Of particular importance is the fact that the angle θ₁ betweenmagnetic vector 24 and the anisotropy direction (axis 23) is equal toangle θ₁ between magnetic vector 25 and axis 23 because of theanisotropy symmetry in the two sub-layers. In interval t₃, the Hy fieldreturns to zero but a current in the second write line 22 continues toexert a field (Hx) aligned in the +x direction. As a result, themagnetic vectors 24, 25 rotate once more in a clockwise direction andare now past their anisotropy hard axis instability points. Note thatthe +x axis bisects the angle formed by the magnetic vectors 24, 25.During the final interval (t₄), there is no current in either of thewrite lines 21, 22 and the magnetic vectors 24, 25 rotate clockwise tothe nearest easy axis position. Note that the magnetic vectors 24, 25have each rotated 180° compared with their original position (memorystate “0”) to effectively switch the memory state for the MTJ cell 20 toa “1”. Each time the sequence t₀-t₄ is followed, the memory state of theMTJ cell 20 changes from a “1” to a “0” or vice versa, depending on theinitial direction of magnetic vectors 24, 25.

FIG. 3 a shows a cross-sectional view of a toggle MTJ cell 43 accordingto one embodiment of the present invention. The toggle MTJ cell 43 isformed between a bit line 40 and word line 41 in an MRAM structure (notshown). It should be understood that the positions of the bit line andword line could be reversed. In one aspect, the toggle MTJ cell 43 iscomprised of a seed layer 42, AFM layer 44, pinned layer 45, tunnelbarrier 46, free SAF layer 47, and cap layer 51 that are sequentiallyformed on the bit line 40. All layers in the MTJ cell 43 are typicallysputter deposited in an Anelva sputter deposition system or the likethat has sputter deposition chambers and at least one oxidation chamber.

The seed layer 42 may be made of NiFeCr, NiCr, Ta, Ru, or laminatedfilms thereof that promote uniform and densely packed growth insubsequently formed layers. Above the seed layer 42 is ananti-ferromagnetic (AFM) layer 44 which may be comprised of PtMn, NiMn,OsMn, IrMn, RuMn, RhMn, PdMn, RuRhMn, or PtPdMn and is used to pin themagnetization direction in an overlying ferromagnetic (pinned) layer 45.

The pinned layer 45 is preferably comprised of one or more of Ni, Co,and Fe or an alloy thereof and has a thickness between about 10 and 200Angstroms. The magnetization direction of the pinned layer 45 may be setalong the MTJ cell's easy axis (not shown). Optionally, the pinned layer45 may be a synthetic anti-parallel pinned (SyAP) layer in which twoferromagnetic layers (not shown) such as CoFe of slightly differentthicknesses are separated by a thin Ru, Rh, or Ir coupling layer thatmaintains strong anti-parallel magnetic coupling between the twoferromagnetic layers as appreciated by those skilled in the art. Inother words, the SyAP pinned layer has a sandwich configuration in whichthe magnetization direction of a lower ferromagnetic (AP2) layer may befixed along the easy axis by the AFM layer 23. An upper ferromagnetic(AP1) layer has a magnetization direction anti-parallel to that of theAP2 layer that results in a small net magnetic moment for the pinnedlayer along the same axis as the AP2 magnetic moment. The SyAPconfiguration minimizes the stray field from the pinned layermagnetization.

A tunnel barrier layer 46 also known as an insulating layer is disposedon the ferromagnetic (pinned) layer 45. The tunnel barrier layer 46 maybe formed by sputter depositing an Al layer on the pinned layer and thenoxidizing to form an AlO_(x) layer with a thickness of about 5 to 15Angstroms before the remaining MTJ layers are sputter deposited.Alternatively, the tunnel barrier layer 46 may be made of TiO_(x),HfO_(x), MgO, or a lamination of one or more of the aforementionedoxides.

A key feature of the present invention is the SAF free layer 47 that inone embodiment has a configuration comprised of a first sub-layer 48, acoupling layer 49, and a second sub-layer 50 formed sequentially on thetunnel barrier layer 46. In the present invention, the two sub-layers48, 50 have the same magnetic moments (one parallel and the otheranti-parallel to the easy axis of the MTJ cell 43) but theiranisotropies are intentionally selected to be different. Thus, the netmagnetic moment of the SAF free layer 47 is essentially zero. A cappinglayer 51 made of a conductive material such as Cu, Ru, Ta, TaN, W or acomposite layer is formed on the second sub-layer 50 to complete thetoggle MTJ cell 43 and typically has a thickness of about 50 to a fewhundred Angstroms. A plurality of MTJ cells in an MRAM array is formedfrom the stack of layers 42-51 by a well known photoresist patterningand etching sequence.

Different anisotropies for the two sub-layers in SAF free layer 47 canbe achieved by selecting different materials for first sub-layer 48 andsecond sub-layer 50. In particular, a material having a largeranisotropy such as CoFeB, CoNiFe, or multi-layers such as CoFeB/NiFe,CoNiFe/NiFe, or NiFe/Ru/NiFe are chosen for the first sub-layer 48. Thefirst sub-layer 48 has a thickness between 15 and 100 Angstroms and itsuni-axial anisotropy is preferably between 10 and 30 Oersted (Oe). Whena NiFe/Ru/NiFe multilayer configuration is employed as the firstsub-layer 48, the net magnetic moment is adjusted to be equal to thesecond sub-layer 50. Typically, the Ru coupling layer thickness is about8 Angstroms to afford very large anti-ferromagnetic coupling between thetwo sub-layers 48, 50. A soft magnetic material with a small anisotropysuch as Ni_(˜0.8)Fe_(˜0.2) and having a thickness of about 15 to 100Angstroms is selected for the second sub-layer 50. The uni-axialanisotropy of the second sub-layer 50 is preferably 0 to 8 Oe or atleast 5 Oe less than that of the first sub-layer 48.

In FIG. 3 b, another embodiment of the present invention is depictedwherein the SAF free layer 47 is comprised of four sub-layers and threecoupling layers. The first sub-layer 90 is disposed on the tunnelbarrier layer 46 and may have a magnetic moment (vector) aligned alongthe easy axis 32 or optionally along the x-axis (not shown). There is afirst coupling layer 91 on the first sub-layer 90 and a second sub-layer92 formed thereon that is strongly anti-parallel coupled to the firstsub-layer. The first and second sub-layers 90, 92 preferably have thesame magnetic moments and anisotropies. Above the second sub-layer 92 isa second coupling layer 93 that may be made of the same material as inthe first coupling layer 91 such as Ru. There is a stack comprised of athird sub-layer 94, a third coupling layer 95, and a fourth sub-layer 96sequentially formed on the second coupling layer 93. The third couplinglayer enables anti-parallel coupling between the third and fourthsub-layers 94, 96. The magnetic vectors of the first and thirdsub-layers 90, 94 are aligned in the same direction along the (+)direction of the easy axis 32, for example, while the magnetic vectorsfor second and fourth sub-layers 92, 96 are in the opposite directionalong easy axis.

An important requirement is that the anisotropy for the first sub-layer90 or fourth sub-layer 96 be greater than the anisotropy for theremaining three sub-layers. In one aspect, the fourth sub-layer 96 mayhave the same composition as described previously for sub-layer 48 andsub-layers 90, 92, 94 are preferably made of a soft magnetic materialsuch as Ni_(˜0.8)Fe_(˜0.2) or the like. Alternatively, the firstsub-layer 90 may have the same composition as described previously forsub-layer 48 and sub-layers 92, 94, 96 are made of Ni_(˜0.8)Fe_(˜0.2) orthe like. However, all four sub-layers 90, 92, 94, 96 have equalmagnetic moments in terms of magnitude.

Referring to FIG. 4, a diagram shows how the magnetic vectors for twosub-layers in a SAF free layer according to one embodiment of thepresent invention rotate in response to an applied magnetic field thatcan be generated with current in bit line 40 and word line 41. In thisexample, magnetic vector 30 for the first sub-layer 48 in the toggle MTJcell 43 is aligned along the easy axis 32 (negative direction) andmagnetic vector 31 for the second sub-layer 50 is oriented in theopposite direction along the easy axis 32 during a quiescent state. Whena magnetic field H_(A1) is applied at a large angle θ₂ away from theanisotropy direction (easy axis 32), the magnetizations of the twosub-layers rotate to form a scissoring structure facing the appliedfield H_(A1). Magnetic vector 30 rotates clockwise 26 to a positionrepresented by vector 30 a and magnetic vector 31 undergoes a clockwiserotation 27 to a position represented by vector 31 a. Since the magneticvectors 30 a, 31 a have not moved beyond their anisotropic hard axisinstability points, they will return back to their original positions(vectors 30, 31) when the applied field is removed. There is no changein memory state of the toggle MTJ cell 43 during this process.

Referring to FIG. 5, a diagram shows how the magnetic vectors 30, 31rotate in response to an applied magnetic field when their originalorientations in a quiescent state are 180° opposite their respectivepositions in FIG. 4. Thus, if the positions of magnetic vectors 30, 31in FIG. 4 represent a memory state “0”, then the positions of vectors30, 31 in FIG. 5 would represent a memory state “1”. With an appliedfield H_(A1) at a large angle θ₂ away from the anisotropy direction,magnetic vector 30 has a clockwise rotation 29 to position 30 b andmagnetic vector 31 undergoes a clockwise rotation 28 to position 31 b.Again, when the field is reduced to zero, the magnetization of the twosub-layers 48, 50 will revert back to the original positions (magneticvectors 30, 31) and the memory state of the toggle MTJ cell 43 willremain unchanged. This behavior is similar to what is observed for aconventional toggle cell design as depicted in FIG. 2. For example, ifthe applied field Hy during interval t₁ in FIG. 2 were reduced to zerowithout proceeding to interval t₂, then the magnetic vectors 24, 25 forthe two sub-layers would revert to their original positions depicted ininterval t₀ and the memory state is not changed.

Referring to FIG. 6, a diagram shows how the magnetic vectors for twosub-layers in a SAF free layer of one embodiment of the presentinvention rotate in response to a magnetic field H_(A2) that is appliedalong the anisotropy direction or at a small angle θ₃ away from the easyaxis 32. Magnetic vector 30 for the first sub-layer 48 in the toggle MTJcell 43 is aligned along the easy axis 32 (arbitrarily assigned to thenegative direction) and magnetic vector 31 for the second sub-layer 50is oriented in the opposite direction along the easy axis 32 during aquiescent state. When the magnitude of the magnetic field H_(A2) isbeyond a critical value called the spin-flop field, Hsf, both magneticvectors of the two sub-layers 48, 50 rotate to form a scissoringstructure facing towards the applied field H_(A2).

In the example shown in FIG. 6, the applied field H_(A2) has a directionat an angle θ₃ away from the (+) easy axis 32 direction. In aconventional toggle mode (FIG. 2) when a field is applied along theanisotropy direction as in interval t₂, the resulting magnetizationangles θ₁ of the magnetic vectors 24, 25 are equal with respect to theanisotropy direction. A different result occurs in the toggle MTJ cell43 of the present invention because the magnetization angles of the twosub-layers 48, 50 with respect to the anisotropy direction (easy axis32) are significantly different. In other words, the applied fieldH_(A2) causes magnetic vector 30 to rotate clockwise 33 to a positionrepresented by magnetic vector 30 c at an angle θ₄ from the anisotropydirection and magnetic vector 31 undergoes a clockwise rotation 34 to anangle θ₅ away from the easy axis 32 wherein θ₄ is substantially smallerthan θ₅. The magnetic vector 30 c of sub-layer 48 is much closer to theanisotropy direction than the magnetic vector 31 c for sub-layer 50.This condition occurs because the anisotropy for the first sub-layer 48is much larger than the anisotropy for second sub-layer 50. In thiscase, when the magnetic field is reduced to zero, the magnetization offirst sub-layer 48 moves to the nearby anisotropy direction along the(+) easy axis direction while the magnetization of second sub-layer 50rotates to the direction opposite that of the first sub-layer 48 due toanti-ferromagnetic coupling. The new positions of magnetic vectors 30,31 are 180° opposite to their starting positions before field H_(A2) wasapplied and this represents a switch in memory state for MTJ cell 32from a “0” to a “1”.

Referring to FIG. 7 a, the diagram shows how the magnetic vectors 30, 31rotate in response to an applied magnetic field when their originalorientations in a quiescent state (memory state “1”) are 180° oppositetheir original position in FIG. 6. Again, the applied field H_(A2) has adirection along the (+) easy axis 32 direction or at a small angle θ₃away from the easy axis and the resulting magnetization angles of thetwo sub-layers 48, 50 with respect to the anisotropic direction are notequal but significantly different. Here, the applied field H_(A2) causesmagnetic vector 30 to rotate clockwise 36 to a position represented bymagnetic vector 30 d at an angle θ₇ from the easy axis and magneticvector 31 undergoes a clockwise rotation 35 to an angle θ₆ away from theeasy axis wherein θ₇ is substantially smaller than θ₆. The magneticvector 30 d of sub-layer 48 is much closer to the anisotropy direction(easy axis 32) than the magnetic vector 31 d for sub-layer 50. When themagnetic field is reduced to zero, the magnetization of first sub-layer48 moves to the nearby anisotropy direction along the (+) easy axisdirection while the magnetization of second sub-layer 50 rotates to thedirection opposite that of the first sub-layer 48 due toanti-ferromagnetic coupling. Thus, the original memory state “1” for thetoggle MTJ cell 32 is retained since the magnetic vectors 30, 31 returnto their original positions before H_(A2) was applied.

In another embodiment (not shown), the field H_(A2) may be applied at asmall angle θ₃ away from the (−) easy axis 32 direction when themagnetic vector 30 is in the (−) easy axis 32 direction in a quiescentstate (memory state “0”). In that case, the magnetic vector 30 wouldrotate clockwise to an angle θ₇ slightly away from the (−) easy axis.Meanwhile, under the influence of applied field H_(A2), the magneticvector 31 would rotate clockwise to an angle θ₆ away from the (−) easyaxis 32 direction wherein θ₇ is substantially smaller than θ₆. When theapplied field is removed, the magnetic vectors return to their originaldirections (memory state “0”) because the magnetization of the firstsub-layer 48 moves to the nearby anisotropy direction along the (−) easyaxis 32 while the magnetization of the second sub-layer 50 rotates tothe direction opposite that of the first sub-layer 48 due toanti-ferromagnetic coupling.

In yet another embodiment (not shown), the field H_(A2) may be appliedat a small angle θ₃ away from the (−) easy axis 32 direction when themagnetic vector 30 is in the (+) easy axis 32 direction in a quiescentstate (memory state “1”). In that case, the magnetic vector 30 wouldrotate clockwise to an angle θ₄ slightly away from the (−) easy axis.Meanwhile, under the influence of applied field H_(A2), the magneticvector 31 would rotate clockwise to an angle θ₅ away from the (−) easyaxis 32 direction. When the applied field is removed, the magnetizationof the first sub-layer 48 moves to the nearby (−) anisotropy directionalong (−) easy axis 32 while the magnetization of the second sub-layer50 rotates to the direction opposite that of the first sub-layer 48 dueto anti-ferromagnetic coupling. As a result, the memory state switchesto a “0”.

The write process for the toggle MTJ cell 43 can be summarized asfollows: when the field is applied along the easy axis or at a smallangle away from the easy axis in a direction that is essentiallyopposite that of the magnetic vector for the first sub-layer with higheranisotropy and the field exceeds the spin-flop field for the SAF freelayer 47, then the memory state will be changed from a “1” to a “0” orfrom a “0” to a “1” after the field is removed. For those toggle MTJcells 43 wherein the magnetic vector for the first sub-layer with higheranisotropy is in the same direction as the applied field or at an angleof more than about 10 degrees away from the easy axis, the memory statewill not be changed during the aforementioned write process. Thisconclusion is true for a MTJ cell design where the bit line 40 and wordline 41 are aligned at 45° to 60° angles with respect to the easy axis.A different write process is necessary when the easy axis overlays a bitline or word line and will be described in a later section.

A major benefit of the aforementioned write process is that there is noneed to know the memory state before writing to the toggle MTJ cell 43.If writing a “1” to selected cells is desired, then a field is appliedalong or at a small angle away from the (+) easy axis direction. Thosecells having a “1” memory state before the write sequence will retainthat state and those cells having a “0” memory state before the writesequence will be changed to a “1” by the write process. Likewise, ifwriting a “0” to selected cells is desired, then a field is appliedalong or at a small angle away from the (−) easy axis direction. Thosecells having a “0” memory state before the write sequence will retainthat state and those cells having a “1” memory state before the writesequence will be changed to a “0” by the write process. This isadvantageously different than in a conventional toggle cell design whereall memory states are changed independent of their quiescent statebefore the write cycle is initiated.

In FIG. 7 b, a diagram is shown that demonstrates the rotation ofmagnetic vectors in an embodiment with four sub-layers in the SAF freelayer such as depicted in FIG. 3 b. The applied field H_(A2) and anglesof rotation θ₆ and θ₇ are the same as described with respect to FIG. 7a. In this case, the vectors 90 a, 94 a indicate the orientations of thesub-layers 90, 94, respectively, and the magnetic vectors 92 a, 96 aindicate the orientations of the sub-layers 92, 96, respectively in aquiescent state that may represent memory state “1”.

When the applied field H_(A2) has a direction along the (+) easy axis 32direction or at a small angle θ₃ away from the easy axis, the resultingmagnetization angles of the two sub-layers 90, 94 with respect to theanisotropic direction are substantially different than those ofsub-layers 92, 96. In this example, sub-layer 90 has a larger anisotropythan the sub-layers 92, 94, 96. Magnetic vectors 90 a, 94 a rotateclockwise 36 to positions represented by magnetic vectors 90 b, 94 b,respectively, at an angle θ₇ from the easy axis. Meanwhile, magneticvectors 92 a, 96 a undergo a clockwise rotation 35 to positions 92 b, 96b, respectively, at an angle θ₆ away from the easy axis wherein θ₇ issubstantially smaller than θ₆. The magnetic vector 90 b of sub-layer 90is much closer to the anisotropy direction (easy axis 32) than themagnetic vectors 92 b, 96 b for sub-layers 92, 96. When the magneticfield is reduced to zero, the magnetization of first sub-layer 90 (andsub-layer 94) move to the nearby anisotropy direction along the (+) easyaxis direction while the magnetization of sub-layer 92 (and sub-layer96) rotate to the direction opposite that of the sub-layer 90 andsub-layer 94, respectively, due to anti-ferromagnetic coupling. Thus,the original memory state “1” for the toggle MTJ cell 43 is retainedsince the magnetic vectors 90, 92, 94, 96 return to their originalpositions before H_(A2) was applied.

Those skilled in the art will appreciate that the toggle MTJ cell 43having four sub-layers 90, 92, 94, 96 in the SAF free layer has memorystates “0” or “1” that may be retained or switched depending on theinitial orientation of the magnetic vectors for the various sub-layersand the direction and angle of the applied field. In addition to FIG. 7b, other magnetic responses to an applied field at a small angle awayfrom the anisotropy direction are similar to those shown in FIGS. 4-6except that two magnetic vectors (for two sub-layers) lie along eachdirection of the easy axis.

Those skilled in the art will also appreciate that the present inventionalso encompasses an embodiment wherein there are a plurality of “n”magnetic sub-layers and “n−1” coupling layers in the free SAF layer 47of toggle MTJ cell 43 wherein “n” is an even number. This configurationhas an anti-ferromagnetic coupling layer between adjacent magneticsub-layers. Thus, there are n/2 magnetic sub-layers having a magneticmoment along the (−) easy axis direction and n/2 magnetic sub-layerswith a magnetic moment along the (+) easy axis direction for a netmagnetic moment of 0 in the SAF free layer. In this case, either theinner magnetic sub-layer on the tunnel barrier layer or the outermagnetic sub-layer adjacent to the cap layer has a higher anisotropythan the remaining magnetic sub-layers.

Referring to FIG. 8, a simulation result is shown for the spin-flopfield as a function of the field angle away from the easy axis. For theexamples in FIG. 6 and 7 a, the field angle is θ₃. In the simulation,the coupling field of the two sub-layers in the SAF free layer is takento be 20 times Hk, the anisotropy of the first sub-layer. Note that onlywhen the applied field is within a small angle where θ₃ is less thanabout 10°, the magnetizations of the two sub-layers can be written witha reasonably small write field. Otherwise, when field angle θ₃ exceeds acertain value, the applied field must be very large to almost saturatethe sub-layer magnetizations in order to change their states.

The present invention is also a method of selectively writing to one ormore toggle MTJ cells in an MRAM structure. Referring to FIG. 9, theeasy axis 32 of the SAF free layer 47 in toggle MTJ cell 43 is parallelto the bit line 40 and normal to the word line 41 according to oneembodiment of the present invention. Although the MTJ cell 43 is shownas circular from a top-down view, it may optionally be shaped in theform of an ellipse, eye, or rectangle, for example.

In FIG. 10, a top view of a section of a MRAM containing a plurality oftoggle MTJ cells 43 is shown wherein the easy axis of the SAF free layerhas the same configuration as described with respect to FIG. 9. Notethat write word lines 41 a-41 d and segmented bit lines 40 a-40 d arethinned to simplify the drawing. In this example, toggle MTJ cells 43along write word line 41 c will be selectively written. For the writeprocess, a segmented bit line algorithm may be used as described in U.S.Pat. No. 6,335,890.

Referring to FIG. 11, a diagram illustrates how a bipolar current in theword line and a current in the bit line are applied to write a “1” inthe selected toggle MTJ cells 43 also referred to as “bits”. The writesequence may be divided into five time intervals I₁ through I₅ with aword line current 60 depicted along the same time axis as a bit linecurrent 61. During interval I₁, there is no current pulse in eitherwrite word line 41 c or in the segmented bit lines 40 a-40 d. A firstcurrent pulse 60 a in the word line current 60 is applied with a certainamplitude during interval I₂ but there is no current pulse during thesame time period in the bit line current 61. This first word linecurrent pulse 60 a effectively writes “1” to all of the selected bitsalong write word line 41 c. During interval I₃, there is no currentpulse in word line current 60 but near the end of the interval I₃, a bitline current pulse 61 a is applied. If the in the first word linecurrent pulse 60 a is a positive pulse, then the bit line current pulse61 a is also positive. Otherwise, if the first word line current pulse60 a is negative, the bit line current pulse 61 a is also negative.

Moving to interval I₄, a second current pulse 60 b in word line 41 c isapplied and is opposite that of bit line current pulse 61 a thatcontinues through interval I₄. Therefore, if first word line currentpulse 60 a is a positive pulse, the second current pulse 60 b is anegative pulse. Preferably current pulses 60 a, 60 b are equal inamplitude. Furthermore, the sum of the fields produced by current pulses60 b, 61 a must be greater than a certain value so that the resultingfield has an angle away from the easy axis of at least 10 degrees. Notethat the bit line current pulse 61 a overlays with the second word linecurrent pulse 60 b during interval I₄ and this yields a large appliedfield angle away from the easy axis. Because of this large angle, thespin-flop field of the SAF free layer is greatly increased to beyond theapplied current field thereby avoiding writing a “0”. During intervalI₅, the bit line pulse 61 a is removed and there is no word line pulse.Thus, the memory state of the toggle MTJ cell is not changed duringinterval I₄ and the selected bits retain the memory state “1” writtenduring interval I₂.

Referring to FIG. 12, the previously described word line current pulsesequence and an alternative bit line current is employed to write a “0”memory state in the selected bits along write word line 41 c. The wordline current pulse sequence remains the same as previously describedthrough all five time intervals I₁-I₅. In this case, no bit line currentpulse is applied so that the second word line current pulse 60 bessentially writes a “0” in the selected bits during interval I₄.

In another embodiment depicted in FIG. 13, a selective toggle MTJ cell73 may be formed at an intersection region of a bit line and word linewherein the bit line 70 and word line 71 have the same angle α of from45° to about 60° away from the cell's easy axis 72. The cell's easy axismeans the anisotropies for the two or more sub-layers in the SAF freelayer (not shown) are aligned along said axis. From a top view, thetoggle MTJ cell 73 may have a circular shape or may have anisotropyinduced by an elliptical, rectangular, or eye shape, for example,wherein the long axis is typically the easy axis. In this scheme, eachtime a sufficient field along the easy axis (+) or (−) direction isgenerated by applying a current in the word line and bit line, theselected bit between the word line and bit line is written.

In FIG. 14, the sum of the fields generated by both word line currentand bit line current during the write process is shown for theembodiment wherein the bit line 70 and word line 71 have the same angleα of from 45° to about 60° away from the cell's easy axis 72. The sum ofthe fields is larger than the SAF free layer's spin-flop field withsufficient margin and is parallel to the easy axis 72 (along the “+x”direction) for writing “1” and along the “−x” direction for writing “0”.It should be understood that the applied field from each write line isgenerated orthogonal to the direction of the current in the write line.

In one aspect according to FIG. 15, the word line current and bit linecurrent referred to in FIG. 14 are matched throughout the entire writesequence and reducing the current pulse back to zero comprises a steadylinear decrease in a so-called “longer current falling time” approach.The time sequence is divided into four intervals Ia-Ib to demonstratethis method. In interval Ia, neither word line current 74 nor bit linecurrent 75 have a pulse. During interval Ib, a positive (or negative)word line pulse 74 a and a matching bit line pulse 75 a are applied. Theamplitude of the pulses 74 a, 75 a are equal and sufficient to generatefields 80, 81 or 82, 83 (FIG. 14) that write the memory state of theselected toggle MTJ cells. Moving to interval Ic that typically lastsfrom about 1 to 10? nanoseconds, the word line current pulse 74 b isreduced to zero along a linear slope. Likewise, bit line current pulse75 b is matched to pulse 74 b and decreases to zero at the same ratewithin interval Ic. During interval Id, both word line current 74 andbit line current 75 have no pulse.

Referring to FIG. 16, an alternative selective write process is shownthat is the same as in FIG. 15 except for interval Ic where the wordline current and matching bit line current pulses are reduced in amulti-step falling edge process wherein a plurality of steps is used toreduce the word line and bit line pulses back to zero. In other words,the amplitude of the pulse is reduced for a certain time and then heldconstant for a certain time within each of the plurality of steps. Inthe example shown in FIG. 16, the word line pulse has three successivesteps 74 b 1, 74 b 2, 74 b 3 in interval Ic wherein the order ofamplitude of the word line pulse within each step is 74 b 1>74 b 2>74 b3>0 and bit line pulse has three successive steps 75 b 1, 75 b 2, 75 b 3wherein the order of amplitude is 75 b 1>75 b 2>75 b 3>0. Moreover, theamplitude and duration of each of the matching steps are equivalent suchthat 74 b 1=75 b 1, 74 b 2=75 b 2, and 74 b 3=75 b 3.

One advantage of the present invention is that the toggle MTJ celldescribed herein does not have half-selected issues as in traditionalMRAMs and therefore the written memory states are more stable. A primaryadvantage over a conventional toggle cell design is that the selectedMTJ cells can be written without first reading to determine theirmagnetic states. This result is achieved by designing differentanisotropies in the two or more sub-layers of the SAF free layer and byapplying a novel write process that leads to a significant reduction inwrite cycle time and is highly desirable for high speed applications.

While this invention has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this invention.

1. A magnetic random access memory (MRAM) structure having a toggle MTJcell wherein a synthetic anti-ferromagnetic (SAF) free layer formedadjacent to a tunnel barrier layer is comprised of a stack that has afirst magnetic sub-layer disposed on the tunnel barrier layer, acoupling layer on the first magnetic sub-layer, and a second magneticsub-layer on the coupling layer, said magnetic sub-layers have the samemagnetic moment but the first magnetic sub-layer has a greateranisotropy than the second magnetic sub-layer.
 2. The MRAM structure ofclaim 1 wherein the first magnetic sub-layer having a greater anisotropyis made of CoFeB or CoNiFe, or is a multilayer comprised of CoFeB/NiFe,CoNiFe/NiFe, or NiFe/Ru/NiFe.
 3. The MRAM structure of claim 2 whereinthe NiFe/Ru/NiFe multilayer has a net magnetic moment equal to thesecond magnetic sub-layer and the Ru layer has a thickness of about 8Angstroms to enable very large anti-ferromagnetic coupling between theNiFe layers.
 4. The MRAM structure of claim 2 wherein the first magneticsub-layer has a uni-axial anisotropy between about 10 and 30 Oersted(Oe).
 5. The MRAM structure of claim 1 wherein the second magneticsub-layer is comprised of a soft magnetic material that isNi_(˜0.8)Fe_(˜0.2) and has a uni-axial anisotropy of about 0 to 8 Oe. 6.A magnetic random access memory (MRAM) structure having a toggle MTJcell wherein a synthetic anti-ferromagnetic (SAF) free layer formedadjacent to a tunnel barrier layer is comprised of a stack of layerscomprising: (a) a first magnetic sub-layer formed on the tunnel barrierlayer; (b) a first coupling layer on the first magnetic sub-layer; (c) asecond magnetic sub-layer formed on the first coupling layer; (d) asecond coupling layer on the second magnetic sub-layer; (e) a thirdmagnetic sub-layer formed on the second coupling layer; (f) a thirdcoupling layer on the third magnetic sub-layer; and (g) a fourthmagnetic sub-layer on the third coupling layer wherein said fourmagnetic sub-layers have equal magnetic moments and the first or fourthmagnetic sub-layer has a larger anisotropy than the remaining magneticsub-layers.
 7. The MRAM structure of claim 6 wherein said remainingsub-layers are made of a soft magnetic material that has a uni-axialanisotropy between about 0 and 8 Oe.
 8. The MRAM structure of claim 6wherein the magnetic sub-layer with a larger anisotropy has a uni-axialanisotropy of about 10 to 30 Oe and is made of CoFeB or CoNiFe, or is amultilayer comprised of CoFeB/NiFe, CoNiFe/NiFe, or NiFe/Ru/NiFe, saidRu layer has a thickness of about 8 Angstroms to allow very largeanti-ferromagnetic coupling between the NiFe layers.
 9. The MRAMstructure of claim 6 wherein each adjacent magnetic sub-layers areanti-ferromagnetic coupled, and the first and second magnetic sub-layersare strongly anti-parallel coupled.
 10. The MRAM structure of claim 6wherein the coupling layers are made of Ru, Rh, or Ir and have athickness of about 8 Angstroms.
 11. The MRAM structure of claim 6wherein the magnetic sub-layers have a thickness between about 15 and100 Angstroms.